1. Field of the Invention
The present invention relates to a radiation image storage panel employed in a radiation image recording and reproducing method utilizing a stimulable phosphor, and a process for the preparation of said panel. More particularly, the invention relates to a radiation image storage panel comprising a support and a phosphor layer provided thereon which comprises a binder and a stimulable phosphor dispersed therein, and a process for the preparation of said panel.
2. Description of the Prior Art
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt material and a radiographic intensifying screen.
As a method replacing the above-described radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. In the radiation image recording and reproducing method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the conventional radiography. Accordingly, the radiation image recording and reproducing method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when excited with stimulating rays such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or having radiated from an object is absorbed by the phosphor layer of the radiation image storage panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating (scanning) the panel with stimulating rays. The stimulated emission is then photoelectrically detected to obtain electric signals, so as to reproduce a visible image from the electric signals.
Such phosphor layer of the radiation image storage panel is conventionally prepared by procedures of adding stimulable phosphor particles and a binder to an appropriate solvent to prepare a homogeneous coating dispersion comprising phosphor particles dispersed in a binder solution, applying the coating dispersion onto a support using a doctor blade, a roll coater, etc., and heating a layer of the coating dispersion to dryness. Otherwise, the phosphor layer is initially formed on a sheet by applying the coating dispersion thereonto and then the phosphor layer is combined with the support. The phosphor particles are uniformly dispersed in the prepared phosphor layer, and therefore, the surface of the phosphor layer is provided with finely protruded and depressed portions because of the appearance of phosphor particles.
To protect the stimulable phosphor on the surface of the phosphor layer from chemical deterioration or physical shock and to make the panel have a smooth surface, a protective film is usually provided on the surface of the phosphor layer by applying a solution of transparent polymer material thereonto or combining a previously prepared transparent film with the phosphor layer using an adhesive agent.
It is desired for the radiation image storage panel employed in the radiation image recording and reproducing method to have a high sensitivity and to provide an image of high quality (high sharpness, high graininess, etc.), as well as a radiographic intensifying screen employed in the conventional radiography.
However, when the radiation image storage panel has a protective film, the light (stimulating rays) tends to be scattered on the interface of the phosphor layer and the protective film. Particularly in the case that the protective film is provided on the phosphor layer via an adhesive layer, the scattering of stimulation rays occurs on each interface among the phosphor layer, adhesive layer and protective film. Thus, the quality of the resulting image is apt to be deteriorated.
Further, the efficiency in the detection of light emitted by the phosphor on the side of the panel surface (surface of the protective film) is reduced because the emitted light is also scattered on the interface(s), so that the sensitivity of the panel decreases.
The radiation image storage panel, generally, is used repeatedly. When the protective film is provided on the phosphor layer via an adhesive layer, the adhesive layer tends to mechanically or thermochemically deteriorate as the use of the panel is repeated. As a result, the protective film has a tendency of peeling off the panel. In other words, the bonding strength is not so sufficient between the phosphor layer and the protective film and the panel is not sufficiently resistant to the repeated use.
More in detail, the radiation image storage panel is repeatedly used in a cyclic procedure comprising steps of: exposing the panel to a radiation (recording a radiation image thereon), irradiating the panel with stimulating rays (reading out the recorded radiation image therefrom) and irradiating the panel with light for erasure (erasing the remaining radiation image therefrom). The panel is transferred from a step to the subsequent step through a transfer system and piled on other panels to be stored after one cycle is finished.
The radiation image storage panel is furthermore apt to be charged on its surface because the contact and friction between panels repeatedly take place in the continuous cycle comprising transfering and piling of the panel. The static electrification on the panel brings about the adhesion with another panel to transfer two panels together in layers from the piling into the transfer system, whereby the subsequent procedure cannot be normally conducted. The charged surface of the panel is likely to be attached with dust in the air, so that the stimulating rays are also scattered on the dust attached thereon and the quality of the resulting image lowers.